1. Field of the Invention
The present invention relates to an anisotropic composition of matter comprising a nematic triazine derivative with the structure below: ##STR2## In the formulae I above, R.sup.3 represents a positively charged, substituted or unsubstituted heteroaromatic ring linked to the triazine backbone through a nitrogen atom within the ring of R.sup.3. If R.sup.3 is an unsubstituted pyridine, the counterion X.sup.- may be any counterion except Cl.sup.- and OH.sup.-. Otherwise, the counterion X.sup.- may be selected from any counterion. When at least one layer of the composition of matter is coated on a supporting substrate and dried, an optical retardation layer is produced which may be used to improve image quality in a display device. The properties of the composition and the substrate may be selected to form a birefringent retardation component that corrects the phase and polarization states of the display's emitted light to improve image quality at viewing angles away from the normal.
2. Description of Related Art
Low power consumption, light weight, and space saving capabilities make liquid crystal displays useful in many fields of application, such as in computers, avionics, and the like. The use of LCDs in some fields has been limited because the image shown on the display degrades as the viewer moves from a direct front view, normal to the display, referred to as a zero viewing angle, to a side view, or off-normal view, at a nonzero viewing angle. The viewing performance deteriorates as the observer moves away from the normal to the non-zero viewing angles in part because the light guiding property of the cell is compromised by the inherent birefringence of the liquid crystal layer and the slightly elliptical polarization state of light transmitted to the viewer. The birefringence of the liquid crystal layer tends to be more pronounced in super twisted nematic (STN) displays than in twisted nematic (TN) displays.
A number of techniques and materials have been developed to improve display quality for light at both normal incidence and at angles of incidence away from the normal. In many currently available LCD devices, phase difference layers, typically multi-layered polymeric films, are placed in the path of the light transmitted to the viewer. These layers, referred to in the art as retardation or retardation layers, rely on optical anisotropy to modify the optical properties of the light emitted from the display. An optically anisotropic film exhibits differing refractive indices n.sub.x, n.sub.y, and n.sub.z for light polarized in planes parallel to the x, y, and z axes, respectively, wherein the x and y axes are in the plane of the retardation layer, and the z axis is normal to the plane of the layer, with the positive direction pointing away from the layer.
Optically anisotropic layers having refractive indices (n.sub.x, n.sub.y, n.sub.z) for light polarized in planes parallel to the x, y, and z axes, respectively, are classified in various ways. If only one of the refractive indices of the layer, for example n.sub.x, differs from the other two, n.sub.y, and n.sub.z, and the other two refractive indices (n.sub.y, and n.sub.z) are substantially equal, the material is said to be uniaxially birefringent. Moreover, if the unequal refractive index (n.sub.x) is less than the two substantially equal refractive indices (n.sub.y and n.sub.z), the material is negatively birefringent, while if the unequal refractive index (n.sub.x) is greater than the two substantially equal refractive indices (n.sub.y and n.sub.z), the material is positively birefringent.
If the material is in the form of a planar sheet, or plate, and if the optical axis is in the plane of the plate, the plate is referred to as an "A" plate. If the optical axis is perpendicular to the plane of the plate, the material is a "C" plate. If the optical axis is at some intermediate, or oblique, angle to the plane of the plate, the plate is an "O" plate. If the plate is an "A" plate and is negatively birefringent, the plate is an "A negative" plate. Anisotropic films having particular relationships between n.sub.x, n.sub.y, and n.sub.z have been produced by a variety of means. For example, U.S. Pat. No. 5,124,824 discloses a retardationplate comprising a liquid crystal material contained between two solid substrates, forming a structure similar to the liquid crystal device itself. The layer in U.S. Pat. No. 5,526,150 is produced by casting a liquid crystal polymer onto a glass plate and heat treating. The retardation plates described in the '824 and '150 patents are expensive, thick, and heavy.
U.S. Pat. No. 5,380,459 teaches that the refractive indices of polymeric films may be adjusted by stretching the film, aligning side chains with electrical or magnetic fields, or by applying shear stresses to the film surfaces. Phase retardation films produced by holding a stretched polymeric film between belts or rollers to constrain the surface while heating the film are disclosed in U.S. Pat. No. 5,474,731. Oriented liquid crystalline polymer films produced by coating a liquid crystalline polymer onto a stretched polymer film and heat treating to allow self-orientation are disclosed in U.S. Pat. No. 5,132,147. Production of compensating layers by heat treating and stretching of films requires a great deal of time, equipment, and cost, which is a disadvantage in a competitive manufacturing environment. In addition, when no single film or film and coating combination produce the desired values of n.sub.x, n.sub.y, and n.sub.z, lamination of multiple films to achieve the desired overall refractive indices can become a burdensome task. If heat treating of layers is used to produce desired optical properties, other polymeric layers which may be present may be adversely affected by the heat treatment.
While the specific values of the refractive indices n.sub.x, n.sub.y, and n.sub.z necessary to produce a suitable retardation layer for a LCD depend upon the chemical structure of the materials in the display, as well as the process by which the materials are deposited, several useful relationships between n.sub.x, n.sub.y, and n.sub.z are known in the art. For example, U.S. Pat. No. 5,124,824 describes a polystyrene retardation layer in which the benzene rings of the polystyrene are oriented in the direction of the thickness of the film to provide a layer in which n.sub.z is greater than n.sub.x or n.sub.y. Similarly, U.S. Pat. No. 5,380,459 describes polymeric materials having side groups containing aromatic groups or other groups containing multiple bonds. These polymeric materials have an n.sub.z greater than at least one of n.sub.x or n.sub.y, and are stated to improve the off-axis viewing angle of a LCD. U.S. Pat. No. 5,406,396 describes a two-layer compensating film. The first layer has refractive indices satisfying the relationship n.sub.z &gt;n.sub.x =n.sub.y, while the second has a refractive indices such that, when the two layers are combined, results in an overall relationship of n.sub.x .gtoreq.n.sub.z &gt;n.sub.y for the two-layer film. The compensating layer described in U.S. Pat. No. 5,526,150 has, in one example, refractive: indices n.sub.x =1.55, n.sub.y =1.55, and n.sub.z =1.75 , which are stated to significantly increase the viewing angle of a liquid crystal display in one direction. The disk-shaped liquid crystal layer described in U. S. Pat. No. 5,568,290 may be deposited on an anisotropic film to produces a material with an n.sub.z less than either n.sub.x or n.sub.y.
U.S. Pat. No. 2,400,877 describes certain dyestuffs that form, under proper conditions, a linear or thread-like molecular structure. The '877 patent teaches that this structure, referred to in the art as a nematic phase or structure, may be preserved by careful evaporation and diffusion of a solution. The '877 patent further suggests that colorless or weakly colored nematic materials may be used in retarders or fractional wave plates to increase the contrast of displays such as cathode ray tubes (CRTs) and LCDs. However, the materials described in the '877 patent become soap-like, or smectic, before drying, and are not useful in LCD applications.
U.S. Pat. No. 4,031,092 to Strebel describes a class of triazine derivatives that form a nematic phase in dilute aqueous solution. The compounds described in the '092 patent, generally referred to as 1-4,6 di(carboxyanilino)-1,3,5-triazine-2-yl! pyridinium salts, form birefringent, varnish-like films when dried. If applied at a thickness of about 0.1 to 5 microns onto a glass substrate which has been surface oriented by unidirectional rubbing, the films may be used to produce a retardation plate. Films of about 0.40 to about 0.65 microns thickness are stated to provide retardations of about 65.degree. to about 120.degree..
The '092 patent states the birefringent films formed from the 1-4,6 di(carboxyanilino)-1,3,5 triazine-2-yl! pyridinium salts are "by normal illumination . . . unnoticeable and substantially colorless." The Stebel patent further states that "slight residual color of the film may usually be discharged when the film is heated above about 150-200 .degree. C. without greatly changing the birefringence or anisotropy." However, the ammonium hydroxide solutions described in the '092 patent are too dilute to provide sufficiently thick films for LCD applications. When coated at a thickness of greater than 1 .mu.m as required for display applications, the present inventors have discovered that the compounds described in the '092 patent have a yellowish tint which distorts the blue colors in the display and causes the display colors to appear purple. In addition, ammonium chloride crystals appear when the ammonium hydroxide solutions of the pyridinium salts described in the '092 patent are coated and dried on a substrate, which causes significant problems in display applications. The aqueous solutions described in the '092 patent are also quite unstable, and their limited shelf life is not acceptable for commercial applications. The solutions described in the '092 patent may be applied to glass substrates only, and will not wet plastic substrates, which represents a serious limitation. The Strebel patent further requires that the substrate be oriented by a rubbing step which is sometimes difficult to perform consistently in a manufacturing setting and which can limit the types of substrates onto which the material can be coated.
There is therefore a need to produce durable, optically anisotropic layers for a wide variety of display applications that can be adapted to enhance the off-axis viewing performance. The materials described in the '092 patent have certain advantages, but compounds are required which are colorless, more easily manufactured and stored, and may be applied directly to a variety of polymeric substrates without the need for extraordinary processing steps. The present invention has been completed based on these needs